In the handling of various types of web and sheet materials, it is important to be able to accurately position the moving material to ensure that the material remains on track and precisely aligned for various subsequent operations, such as cutting, slicing, printing and the like. Edge detectors which detect the lateral position of the edge of the moving web are utilized in such industries as paper making and converting, where the moving material is paper or nonwoven fibrous webs, in the printing industry, for photographic film manufacturing, and in the plastic packaging and forming industry.
A variety of techniques have been utilized to sense the position of the moving web, including photoelectric sensors in which the amount of interruption of a beam of light by the web is detected, air sensors in which a moving stream of air is directed across the edge of the web and the occlusion of the air is detected, and ultrasonic sensors which direct a beam of ultra-high frequency sound across the edge of the web and detect the amount of occlusion of the beam by the web. These transducers provide an electrical signal which is related to the lateral position of the web, with this signal being utilized to control positioning mechanisms to bring the moving web back to its desired edge position. Ultrasonic edge position detectors have a number of advantages over photoelectric and air transducers, particularly with transparent or translucent web material such as thin paper sheets or transparent plastic, where photoelectric sensors may be difficult or impossible to use.
In an ultrasonic web edge detector, a sound emitting transducer (transmitter) projects a beam of high frequency sound across a gap where it is either received directly by a microphone (receiver) on the other side of the gap or is reflected back to a microphone. As the edge of a web enters the gap, it partially blocks the sound beam, with the sound energy received by the microphone being roughly inversely related to the percentage of occlusion of the sound beam by the web. The relationship between the degree of occlusion and the signal provided by the microphone can be determined for a particular web material and the processing electronics which receives the signal can be adjusted accordingly so that the final control signal is truly proportional to the lateral position of the web edge.
While ultrasonic web detectors enjoy several advantages over other types of edge sensors, various ambient operating conditions can affect the accuracy of the control signals produced by the sensing system. For example, changes in the relative humidity of the ambient air can affect the propagation of the ultrasonic signal and thereby affect calibration, so that a sensor which is properly calibrated on one day may be somewhat off in its readings the next day when the ambient atmosphere has a different relative humidity. Preferably, the edge detector should be relatively insensitive to the elevational position of the web in the gap so that as the web moves toward or away from the receiving transducers because of transient undulations in the traveling web, the sensor does not interpret these motions as changes in the lateral position of the web. Conventional non-pulsed ultrasonic sensors have problems due to the continuous nature of the sensing beam of energy. Reflections of this energy will cause interference from the reflective energy to be sensed in addition to the desired portion of the unblocked beam. These reflections are portions of the ultrasonic energy that have been returned to the detector sensor after bouncing off of objects not in the immediate area of the transducers and can interfere with and greatly reduce the accuracy of the sensors. This reflected energy problem can be reduced by pulsing the ultrasonic signal from the transducer. A particular problem that has been experienced under certain conditions with pulsed ultrasonic transducers is the phenomenon of "ringing", in which the transmitter continues to oscillate after it has received a burst of signal energy near the resonant frequency of the transmitter. Other conditions which can affect the accuracy of the reading from the edge sensor include the temperature of the air, which also affects the sound conduction of the air in the gap, the temperature of the ultrasonic transducers which affects their sensitivity, and air currents in the gap which can cause transient variations in the signal produced by the sensor and which effectively add a "noise" component to the signal of interest.